Hot water tap trickles to nothing.. help?

[quinnster]

We've just bought and moved into a flat, and had the old tank and boiler replaced with an Ideal Isar HE24 condensing combi boiler. They removed the old cold water tank so both hot & cold should be mains pressure.

All is well, except for the bath/shower mixer tap. Hot water comes out quite happily when the hot tap is first turned on, but within a minute the flow rate drops rapidly to a dribble, and then stops altogether, even if the tap is opened fully.
Earlier in the week it had been just dropping to a trickle, but I tried it this evening a couple of times and the flow now stops completely.
The tap makes a vaguely high-pitched noise just before it stops.

Cold water comes out of it fine, and hot water comes out of the other taps in the flat continuously with no stop in flow. It's just the bath mixer hot tap that's at fault, so I don't think it's a boiler problem.
Interestingly, although you can't normally turn two taps on at once with the combi, once the bath hot tap has ground to a halt, the bathroom sink hot tap can be opened and a full flow of hot water comes out, as though the bath tap was not open at all.

Would something simple like changing the washer be worth a try, or should I get the experts in?

 

[ChrisR]

Yes - look at the washer. SOmetimes the squashng they get means that a sideways hole out, is effectively blocked. A bit of thermal expansion then finishes the flow right off. Try trimming the circumference of the washer with a sharp knife.

Could also be that the washer and its support have come loose of course.

 

[quinnster]

Well after some wrestling I managed to get the tap out, and had a look at the washer. It seems to be very flattened and is much thicker at the centre than it is at the edges.. does that sounds like an over-squashed washer?

The washer is held on with a hexagonal nut which I can't get off, simply because I don't have anything small enough to get a decent grip on it, so I'll leave it until I've got some tiny spanners.

 

[ChrisR]

Use an adjustable spanner - I keep a 4" one on my penknife/torch/rad vent key in my pocket!
Pair of pliers should also work.

 

[quinnster]

Managed to borrow some tiny spanners from work, and got the washer off - unfortunately flipping it over made no difference, and the new 'standard sized' 3/8" or 1/2" washers I bought wouldn't quite fit - so I cheated and swapped the insides of the hot and cold taps over.
I hope this is 'allowed' - they look identical, except that the cold side was a bit scaled up.
It all seems to work fine now, though if it goes belly-up again I'll save myself days of grief and just get a plumber in

 

[ChrisR]

- unfortunately flipping it over made no difference, and the new 'standard sized' 3/8" or 1/2" washers I bought wouldn't quite fit -

How the heck should turning it over help as I said the DIAMETER IS TOO BIG!

New washers always look small, but they only have to be big enough

 

[quinnster]

How the heck should turning it over help as I said the DIAMETER IS TOO BIG!

Well the edges of the washer had started to creep over the sides of the valve, so it was too big. I thought that was what you meant!
Turning it over meant that it stopped doing that.
Didn't really want to start cutting bits off it unless I had a replacement - and the replacements I bought didn't seem to fit.

I haven't got a clue how big the washer should be. I don't know if it should have a few mm gap or not around its edges.. that's why I was asking for advice! I can't imagine it's completely the wrong size because the washer in the cold valve is the same size, and last week it was vaguely working.
I know that unless you look at it yourself, you don't know either.

It's probably nothing to do with washers and is a problem with that valve (it takes fewer turns to fully open it than the other valve does).
But since my tap problems aren't very interesting and are resolved anyway, I'll shut up and just say thankyou for your advice